btrfs-progs: show-super: Add option to print superblock at given bytenr

Add '-s <sb_bytenr>' option to show superblock at given bytenr.

This is very useful to debug non-standard btrfs, like debuging the
1st stage btrfs of btrfs-convert.

Signed-off-by: Qu Wenruo <quwenruo@cn.fujitsu.com>
[ minor updates in docs ]
Signed-off-by: David Sterba <dsterba@suse.com>

Documentation/btrfs-show-super.asciidoc

@@ -40,6 +40,10 @@ If several '-i <super_mirror>' are given, only the last one is valid.
 Attempt to print the superblock even if no superblock magic is found.  May end
 badly.
 
+-s <bytenr>::
+specifiy offset to a superblock in a non-standard location at 'bytenr', useful
+for debugging (disables the '-f' option)
+
 EXIT STATUS
 -----------
 *btrfs-show-super* will return 0 if no error happened.

btrfs-show-super.c

@@ -48,6 +48,7 @@ static void print_usage(void)
 	fprintf(stderr, "\t-a : print information of all superblocks\n");
 	fprintf(stderr, "\t-i <super_mirror> : specify which mirror to print out\n");
 	fprintf(stderr, "\t-F : attempt to dump superblocks with bad magic\n");
+	fprintf(stderr, "\t-s <bytenr> : specify alternate superblock offset\n");
 	fprintf(stderr, "%s\n", PACKAGE_STRING);
 }
 
@@ -63,7 +64,7 @@ int main(int argc, char **argv)
 	u64 arg;
 	u64 sb_bytenr = btrfs_sb_offset(0);
 
-	while ((opt = getopt(argc, argv, "fFai:")) != -1) {
+	while ((opt = getopt(argc, argv, "fFai:s:")) != -1) {
 		switch (opt) {
 		case 'i':
 			arg = arg_strtou64(optarg);
@@ -86,6 +87,10 @@ int main(int argc, char **argv)
 		case 'F':
 			force = 1;
 			break;
+		case 's':
+			sb_bytenr = arg_strtou64(optarg);
+			all = 0;
+			break;
 		default:
 			print_usage();
 			exit(1);